1. Field of the Invention
This invention generally relates to the field of liquid crystal displays, and more particularly relates to multi-domain low twist angle liquid crystal cells having wide viewing angles and high stability, and methods of producing such liquid crystal cells, particularly methods of generating multi-directional liquid crystal alignments with a particle beam and scanning mask.
2. Description of Related Art
Typical liquid crystal (“LC”) cells, or pixels, used in liquid crystal displays (“LCDs”) make use of nematic and twisted nematic (“TN”) LC phases, although other LC phases, such as cholesteric and smectic, may also be used. LC molecules are widely used in display applications because of their optical and electromagnetic anisotropy. In particular, LC molecules tend to assume an orientation in which the majority of LC molecules are aligned along their long axis in a preferred direction; this preferred direction is referred to as the LC director orientation. Furthermore, LC molecules adjacent to a surface that has been imparted with an alignment direction (or “alignment orientation”) will tend to assume a director orientation that is parallel to the alignment direction of the surface. Typically, in TN LC cells, an LC medium is disposed between a pair of substrates, wherein each of the substrates has a different alignment direction; typically, the alignment direction of each of the substrates are perpendicular to each other. This difference in alignment directions forces the LC molecules to form a continuous twist arrangement between the pair of substrates. The angle between the alignments on the opposing substrates, through which the LC molecules twist, is referred to as the twist angle. Furthermore, different regions of at least one of the substrates can be aligned in different directions, thereby creating different alignment domains. Typically, LCD operation is based on the ability of light to pass through the LC cell when the LC molecules are in the twisted arrangement and, by applying a field, such as an electric or magnetic field, to the LC cell, the LC molecules can be forced to un-twist, thereby preventing light from passing through the LC cell.
LC molecules in typical twist nematic liquid crystal displays (“TN LCDs”) alter between a planar twist state and a tilted orientation. When the liquid crystal molecules attain the tilted orientation, light from the direction of incidence is subject to various different indexes of reflection. Since the functionality of LCDs is based on the birefringence effect, the transmittance of light will vary with different viewing angles. However, due to such differences in light transmission, optimum viewing of an LCD display is achieved within a limited viewing angle. The limited viewing angle of LCDs is one of the major disadvantages associated with LCDs and is a major factor in restricting the commercial applications for which LCDs could be used.
Several approaches exist for increasing the viewing angles of LCDs, such as in-plane switching (“IPS”), multi-domain vertical alignment, compensation films, and multi-domain TN LC configurations. IPS mode uses comb-like inter-digitized electrodes to apply electrical fields in the plane of the substrates, thereby aligning the liquid crystal molecules along the substrates and providing wide viewing angles for use in wide viewing angle monitors or other applications. However, although IPS provides wide viewing angles, it requires high voltages and has low aperture ratios. In addition, due to the planar electric field structure, IPS mode inherently suffers from severe image sticking. In vertical alignment mode, a multi-domain arrangement is achieved by introducing a protruding structure that forces the liquid crystal molecules to tilt in different directions. However, vertical alignment mode requires an extra photolithography step during fabrication and compensation film is necessary to improve viewing angles.
It is well known in the art that viewing angles of TN LCDs can be improved by dividing each display pixel into two or more LC sub-domains with each sub-domain having a different LC director orientation (i.e., LC alignment direction). Chen et al. (J. Chen, P. J. Bos, J. R. Kelly, SID'97 DIGEST, p. 937) has reported the fabrication of four-domain twist nematic (“4D-TN”) LC cells and LCDs by reverse rubbing and double oblique evaporation techniques in which each domain of the 4D-TN LC cell having normal twist is surrounded by three domains having reverse twist and vice versa, and the twist angle of each domain is 90 degrees. However, if the pixel size is below about 300 microns, it is difficult to stabilize a 4D-TN LC cell at zero field unless the pre-tilt angle is larger than about 25 degrees or the 4D-TN LC cell is stabilized by a polymer matrix. Such a polymer matrix can be formed by field- and UV-induced polymerization of UV-curable diacrylate monomer added to the nematic LC medium; however, the costs are high for materials and manufacturing of LCDs comprised of such polymer-stabilized 4D-TN LC cells. No LC alignment techniques based on rubbing polyimide films exist that can generate a pre-tilt angle larger than about 15 degrees in order to stabilize a 4D-TN LC cell at either zero or high field. Although LC alignment by oblique evaporation of oxides can achieve a pre-tilt angle larger than about 20 degrees, the technique is prohibitively expensive for use in manufacturing and can't be applied to large substrates.
Processes used in the art for fabricating multi-domain TN LC cells, and LCDs comprised of such LC cells, have typically been based on either polyimide rubbing or UV exposure (photo-alignment). However, the cost of such processes is prohibitively expensive due to the need for multiple photolithographic steps between the rubbing steps in order to produce the desired alignment results. In addition, lithographic procedures introduce contamination in the alignment layers and suffer from reliability problems.
Recently, it has been shown that ion beam irradiation can be used to align surfaces for LCD manufacturing. For example, U.S. Pat. No. 5,770,826 (Chaudhari et al.) describes the use of low energy ion beams to impart an alignment direction to the surface of a wide variety of materials. However, the present method of ion beam irradiation used in the art only allows for the fabrication of single domain displays by one-directional alignment. Multi-domain alignment can be accomplished with ion beam irradiation using the same photolithographic techniques that have been used in rubbing processes, but the cost of such procedures is prohibitively expensive.
Therefore a need exists to overcome the problems with the prior art as discussed above, and particularly for multi-domain LC cells, and LCDs comprised of such LC cells, that provide wide viewing angles and are stable at zero and high fields, as well as methods for fabricating such multi-domain LC cells and LCDs, particularly methods that are inexpensive and eliminate the photolithographic steps required for producing multiple alignment directions.